The present invention relates to power supplies for microprocessors acting as electronic control units/controllers (ECUs) and is concerned in particular with the problem of maintenance of ECU operation in the event of unintentional power supply interruptions.
Microprocessors are used widely to control systems that are safety-critical, for example electronic servo braking systems in vehicles, vehicle anti-lock braking systems, vehicle engine management systems, vehicle active suspension systems, vehicle ignition system, and the like.
Since it is common for a controller in a vehicle to experience a very harsh power supply, it is important that such controllers be arranged to have a high immunity to electrical noise generated within, or transmitted to it, by its operational environment.
The supply voltage to an ECU controller is unpredictable, and may contain many glitches and spikes of high energy, fast transient or long duration disturbance. These disturbances constitute noise in the electrical supply system and, if the ECU power supply system is not adequate to cope with such noise, the ECU may give unpredictable results, and degrade system and vehicle performance.
The source of noise in the electrical system of a vehicle may be generated from a wide variety of sources, such as fans, relays, rapid changes in current through inductive loads (spikes), high current transients through the battery source impedance (glitches), alternator and starter motor noise and ignition circuit noise. An ECU power supply must be designed to both tolerate and operate within these worst case (noise) conditions.
For example, in a practical situation, a controller may have a specification to work down to 7.000 volts continuous battery (from a normal 12 volt level), and operate during a condition where the supply to the ECU has a supply interruption where the voltage falls instantaneously to zero, remains at that level for some time, then rises up to 7.000 volts again. The controller needs to be fully functional during this disturbance and so requires a power supply with xe2x80x9chold-upxe2x80x9d capabilities.
The traditional way to achieve hold-up operation for the controller supply is illustrated in FIG. 1 of the attached drawings which shows the input of a voltage regulator 10 coupled to a B+ supply (say 12 volts) via a forward biassed blocking diode D1. Coupled in parallel between the input of the low-drop voltage regulator 10 and the other power supply line 12 are a high power zener diode Z1, and an electrolytic storage capacitor C1. The B+ line normally also includes a current limiting resistor (not shown). The voltage regulator output is on a line 14 and leads to the controller itself (not shown).
This circuit copes with supply interruptions by storing charge within the electrolytic capacitor C1. The energy stored (E) is given by:
E=0xc2x75.C.V2
where
E is in Joules
C is in Farads
V is in Volts
The equation can be expanded to give:
E=0xc2x75.C.(V1xe2x88x92V2)2,
where
V1=Capacitor start voltage before supply interruption
V2=Capacitor end voltage that causes the voltage regulator to be out of regulation.
In order for this circuit to be successful, therefore, the capacitor C1must be physically large enough to cope with supply interruptions from a low start voltage, eg. when cranking the engine. The rated voltage of the capacitor must be at least the clamp voltage of the zener Z1. Furthermore, the rated capacity of the capacitor must be large as V1xe2x88x92V2 is small. Thus, the capacitance value, and hence the physical size of the capacitor, must increase as supply voltage falls. For a constant value of the required energy, the capacitance C must be 2E/V2.
This results in the requirement for a physically large capacitor.
There is known already from GB-A-2262003 an arrangement for providing protection against a drop in supply voltage wherein a supply voltage at an input terminal is applied to an output terminal via a Schottky diode, a capacitor is charged to a voltage greater than the supply voltage by a voltage step-up circuit, and the capacitor is connected to the output terminal when an FET is turned on in response to a circuit sensing that the voltage at terminal has dropped below a predetermined level. The step-up circuit includes a charge pump circuit and a voltage regulating and current limiting circuit. The charge pump circuit charges first and second capacitors, respectively to two and three times the input voltage. The input of regulator is connected to the first capacitor and the second capacitor provides a bias voltage for FET.
This invention relates to the maintenance of ECU operation in the event of unintentional power supply interruptions.
There is therefore a need for a circuit by which the physical size of the capacitor could be smaller for a given performance. As described above, known methods for maintaining ECU operation typically require a physically large capacitor. There is therefore a need for a circuit by which the physical size of the capacitor could be smaller for a given performance.
An object of the present invention is to provide a circuit which controls the input voltage of a voltage regulator at a substantially constant level for the duration of a supply interruption.
In accordance with the present invention, there is provided a voltage supply circuit for an ECU, of the type which uses a capacitor to hold a charge for use in maintaining the supply during temporary supply interruptions, wherein a means is provided for increasing the voltage available for charging the capacitor continuously to a level above that of the supply to enable the stored energy of the capacitor to be boosted, characterised in that the charged capacitor is arranged to be selectively coupled to the input of a voltage regulator when the regulator input voltage falls to a predetermined level as a result of a supply interruption, such as thereafter to maintain the regulator input voltage substantially at that level until the supply interruption is over, and in that a voltage dependent on the input voltage of the regulator is input to a comparator whose output controls the conduction of a path connecting the charged capacitor to the regulator input.
Preferably, the path whose conduction can be controlled comprises a first switching transistor having a control terminal coupled to the output of the comparator, detection of a fall in the input voltage to the voltage regulator causing the comparator to provide an output which results in a control input to the first transistor such as to open the transistor and couple the charged capacitor to the input of the voltage regulator.